1. Field of the Invention
The present application relates generally to systems and methods for determining the orientation of a survey tool within a borehole.
2. Description of the Related Art
Directional drilling for the exploration of oil and gas deposits advantageously provides the capability of generating boreholes which deviate significantly relative to the vertical direction (that is, perpendicular to the Earth's surface) by various angles and extents. In certain circumstances, directional drilling is used to provide a borehole which avoids faults or other subterranean structures (e.g., salt dome structures). Directional drilling is also used to extend the yield of previously-drilled wells by reentering and milling through the side of the previously-drilled well, and drilling a new borehole directed so as to follow the hydrocarbon-producing formation. Directional drilling can also be used to provide numerous boreholes beginning from a common region, each with a shallow vertical portion, an angled portion extending away from the common region, and a termination portion which can be vertical. This use of directional drilling is especially useful for offshore drilling, where the boreholes are drilled from the common region of a centrally positioned drilling platform.
Rotatably steerable drilling systems which are configured to respond to control signals by adjusting the direction of drilling are known in the prior art. Exemplary rotatably steerable drilling systems are disclosed by U.S. Pat. No. 6,659,201, U.S. Pat. No. 6,655,460, U.S. Pat. No. 6,257,356, and U.S. Pat. No. 5,099,927, each of which is incorporated in its entirety by reference herein.
Directional drilling is also used in the context of horizontal directional drilling (“HDD”) in which a pathway is drilled for utility lines for water, electricity, gas, telephone, and cable conduits. Exemplary HDD systems are described by Alft et al. in U.S. Pat. Nos. 6,315,062 and 6,484,818. Such HDD systems typically drill along relatively short distances substantially horizontal to the surface and do not drill very far below the surface. In addition, these HDD systems typically drill holes which reemerge as well.
The pathway of a directionally drilled borehole is typically carefully planned prior to drilling, and the position and direction of the drilling tool is repeatedly determined during the drilling process using surveys to map the pathway of the borehole relative to a fixed set of known coordinates. In certain types of wireline surveys, the drilling of the borehole is periodically halted and a survey tool is lowered into the borehole. In some instances, the drilling assembly (i.e., the drill string and the drilling tool, which includes the drill bit) is removed from the borehole so that the survey tool can be lowered into the borehole. In other types of wireline surveys, the drilling assembly remains in the borehole and the survey tool is lowered into the drilling assembly. As the survey tool is guided along the borehole, it provides information regarding its orientation and location by sending signals through a cable to the surface. This information is then used to determine the pathway of the borehole. The survey tool is then removed from the borehole and drilling is continued, which may require returning the drilling assembly to the borehole if it was removed for the survey. Such wireline surveys thus require extensive time and effort to repeatably stop drilling, insert the survey tool into the borehole, and remove the survey tool (and perhaps the drilling assembly) each time a survey is performed. Since the costs associated with operation of a drilling system can be quite high, any time reductions in borehole surveying can result in substantial cost savings.
In “measurement while drilling” (“MWD”) drilling systems, the survey tool is a component of the drilling system. In such drilling systems, the survey tool can be a component of the drilling tool, typically in proximity to the drill bit, and it remains within the borehole throughout the drilling process. MWD survey measurements of the orientation and location of the MWD survey tool can be made without removing the drilling assembly from the borehole. Typically, MWD survey measurements are taken during periods in which additional drill pipes are connected to extend the drill string and the drilling assembly is substantially stationary, which takes approximately one to two minutes to a few minutes. Use of MWD surveys saves time during operation of the drilling system by eliminating the need to stop the drilling process or to remove and replace the survey tool (and perhaps the drilling assembly) in order to survey the pathway of the borehole. Such MWD drilling systems are known in the art.
In “logging while drilling” (“LWD”) drilling systems, the drilling system includes a survey tool and a logging string having one or more geophysical sensors configured to provide information regarding the geological formations surrounding the borehole at various depths. Examples of geophysical sensors compatible with LWD drilling systems include, but are not limited to, geophones configured to make porosity and/or density measurements using sonar, gamma-ray detectors configured to detect gamma rays from the surrounding geological formations, and resistivity sensors configured to make porosity and/or pore content measurements. In addition, certain LWD logging strings include calipers configured to mechanically sense aspects of the borehole and its casing (e.g., size, amount of wear). In certain instances, the survey tool is a component of the logging string, while in other instances, the survey tool is in proximity to the logging string. Such LWD drilling systems are known in the art.
Gyroscopes, together with accelerometers, have been used in survey tools to measure the azimuth and the inclination of a borehole at a given depth, and the high-side toolface and/or the azimuthal toolface of the survey tool at the same depth. Such measurements typically utilize a minimum number of sensitive gyroscope axes (i.e., the gyroscope provides angular rotation rate information for rotations about these axes). Existing survey tools include only the minimum number of sensitive gyroscope axes to perform the desired measurements, due to the high costs of spinning-wheel and ring-laser gyroscopes.